This invention relates to a semiconductor laser drive and, more particularly, to a semiconductor laser drive of an optical transmitter used in optical communication.
An increase in the speed of communication equipment and computers in recent years has been accompanied by growing demand for communication between apparatus by using light. Accordingly, an optical communication apparatus is required to be small in size, consume little electric power and have only a few locations requiring adjustment.
FIG. 20 is a block diagram showing a semiconductor laser drive in an optical transmitting apparatus according to the prior art. A semiconductor laser 1 emits light when a current which exceeds a threshold value flows through it. A drive current supply unit 2 passes a drive current I.sub.P through the semiconductor laser 1 when an input data signal DT, the level of which alternates in conformity with the "1", "0" logic of data, is smaller than a reference voltage V.sub.ref, and a bias current supply unit 3 passes a prescribed bias current I.sub.B through the semiconductor laser 1. A drive-current controller 4 controls the magnitude of the drive current I.sub.P, and a bias-current controller 5 controls the magnitude of the bias current I.sub.B. The drive-current controller 4 and the bias-current controller 5 each have a variable resistor for adjusting the drive current I.sub.P and bias current I.sub.B, respectively.
The relationship between the optical output of the semiconductor laser 1 and the current value is as illustrated in FIG. 21. According to the characteristic, light is not produced until a threshold-value current I.sub.th is attained. Once a value above the threshold-value current I.sub.th has been reached, the optical power increases in conformity with an increase in current. Accordingly, the bias-current controller 5 controls the bias current in such a manner that the relation I.sub.B .apprxeq.I.sub.th is established, and the drive-current controller 4 controls the drive current I.sub.P in such a manner that a prescribed optical power P.sub.0 is obtained. Further, if the characteristic of the semiconductor laser varies, as indicated by the dashed line in FIG. 21, owing to a variance in the element, the optical power P.sub.0 ' declines. This means that the bias current is controlled so as to establish the relation I.sub.B .apprxeq.I.sub.th ', and the drive-current controller 4 controls the drive current I.sub.P so as to obtain the prescribed optical power P.sub.0.
FIG. 22 is a diagram showing the circuit construction of the semiconductor laser drive according to the prior art, in which components identical with those shown in FIG. 20 are designated by like reference characters. The drive current supply unit 2 includes a differential circuit DFAI composed of field-effect transistors (referred to as "FETs") Q1, Q2 whose respective inputs are the input data signal DT, the level of which alternates in conformity with the "1", "0" logic of data, and the reference voltage V.sub.ref, a source-follower circuit SFL composed of FETs Q3, Q4 for adjusting the level of the differential output, and an output-side differential circuit DFAO composed of FETs Q5, Q6.
The data signal DT enters the gate terminal of the first FET Q1 constructing the differential circuit DFAI on the input side, the constant reference voltage (fixed) V.sub.ref enters the gate terminal of the second FET Q2, the gate terminals of the FETs Q1, Q2 are connected to a power-supply voltage V.sub.SS (negative polarity) via a constant-current source I.sub.0, and the drain terminals of the FETs Q1, Q2 are connected to a power-supply voltage V.sub.DD (ground) via resistors R1, R2 and R3. When the input data signal DT is greater than the reference voltage V.sub.ref, the FET Q1 turns on and the FET Q2 turns off. When the input data signal is smaller than the reference voltage V.sub.ref, the FET Q1 turns off and the FET Q2 turns on.
Several diodes D1-D4 are inserted at the source terminals of the FETs Q3, Q4, which construct the source follower circuit SFL, in order to adjust level. The drain terminals of the FETs Q1, Q2 constructing the differential circuit DFAI on the input side are connected to the gate terminals of the FETs Q3, Q4, respectively, the drain terminals of the FETs Q3, Q4 are connected to the power-supply voltage V.sub.DD (ground), and the source terminals are connected to the power-supply voltage V.sub.SS (negative polarity) via the level adjusting diodes and constant-current sources I.sub.1, I.sub.2, respectively.
The drain terminal of the first FET Q5 constructing the differential circuit DFAO on the output side is connected to the power-supply V.sub.DD (ground), and the drain terminal of the second FET Q6 is connected to the semiconductor laser. The source terminals of the FETs Q3, Q4 of the source-follower circuit SFL are connected to the gate terminals of the FETs Q5, Q6, respectively, and the source terminals of the FETs Q5, Q6 are connected to the power-supply V.sub.SS via a third FET Q7 and a diode D5. The gate terminal of the third FET Q7 is connected to the drive-current controller 4, and the drive current I.sub.P is controlled by the gate-source voltage V.sub.gs.
The bias-current supply unit 3 has a FET Q.sub.8 and a diode D6. The drain terminal of the FET Q.sub.8 is connected to the semiconductor laser 1, the gate terminal of the FET Q8 is connected to the bias-current controller 5, and the bias current I.sub.B is controlled by the gate-source voltage V.sub.gs.
The drive-current controller 4 has a variable resistor VR1 connected between ground and the power-supply voltage V.sub.SS, and the gate-source voltage of the FET Q7 of the drive-current supply unit 2 is adjusted to control the drive current I.sub.P. The bias-current control unit 5 has a variable resistor VR2 connected between ground and the power-supply voltage V.sub.SS, and the gate-source voltage of the FET Q8 of the bias-current supply unit 3 is adjusted to control the bias current I.sub.B.
When the level of the input data signal DT is greater than the reference voltage V.sub.ref (i.e., when data="1" holds), the FET Q1 of the differential circuit DFAI turns on and the FET Q2 turns off. As a result, the FETs Q5, Q6 of the differential circuit on the output side turn on and off, respectively, and therefore the drive current I.sub.P does not flow through the semiconductor laser 1 and no light is produced. On the other hand, when the level of the input data signal DT is less than the reference voltage V.sub.ref (i.e., when data="0" holds), the FETs Q1 and Q2 of the differential circuit DFAI turn off and on, respectively. As a result, the FETs Q5, Q6 of the differential circuit on the output side turn off and on, respectively, and therefore the drive current I.sub.P flows through the semiconductor laser 1 so that light is produced and outputted. In this case, a desired optical power P.sub.0 is obtained by adjusting the variable resistor VR1 of the drive-current controller 4 and the variable resistor VR2 of the bias-current controller 5.
The characteristic of optical power vs. current varies depending upon temperature, as shown in FIG. 23. In order to generate a constant optical power P.sub.0 irrespective of temperature, therefore, it is necessary to control at least one of bias current and drive current in dependence upon a fluctuation in temperature. More specifically, in a case where the optical power P.sub.0 is obtained at ordinary temperature with a bias current of I.sub.B and a drive current of I.sub.P, it is required that the bias current and drive current be made say, I.sub.B ', I.sub.P ', respectively, in order to generate the same optical power P.sub.0 at high temperature. Consequently, in the semiconductor laser drive according to the prior art, the diode D5 having a negative characteristic with respect to temperature is serially connected to the FET Q7 of the drive-current supply unit 2, and the diode D6 having a negative characteristic with respect to temperature is connected to the FET Q8 of the bias-current supply unit 3.
When the temperature rises, the voltage drop across each of the diodes D5, D6 decreases. As a result, the gate-source voltage V.sub.gs of each of the FETs Q7, Q8 becomes larger, there is an increase in the drain current of each FET, namely in the drive current I.sub.B and bias current I.sub.P, and the current flowing into the semiconductor laser 1 increases. As a result, a reduction in optical power which accompanies the rise in temperature is compensated for so as to render the optical power approximately constant. Conversely, when the temperature falls, the voltage drop across each of the diodes D5, D6 increases. Consequently, the gate-source voltage V.sub.gs of each of the FETs Q7, Q8 becomes smaller, there is a decrease in the drive current I.sub.B and bias current I.sub.P, and the current flowing into the semiconductor laser 1 decreases. As a result, a reduction in optical power which accompanies the rise in temperature is compensated for so as to render the optical power approximately constant.
Thus, in the semiconductor laser drive according to the prior art, a variance in the semiconductor laser characteristics is corrected for and a temperature compensating characteristic is imposed upon the drive current and bias current in the drive-current controller 4 and bias-current controller 5, thereby to stabilize the optical power. In order to obtain an excellent transmission characteristic, there is a need for an optical power waveform having a large extinction ratio (the ratio of optical power at "1" to optical power at "0") and an excellent eye aperture characteristic. With the conventional method of driving a semiconductor laser, therefore, the bias current is controlled in such a manner that I.sub.B .apprxeq.I.sub.th is obtained.
A problem which arises is that owing to variance in the threshold-value current I.sub.th and a disparity in the temperature characteristic from one semiconductor laser to another, and depending upon the adjustment of the bias current, the relation I.sub.B .apprxeq.I.sub.th cannot be achieved and there is a deterioration in the extinction ratio of optical power. In addition, the optical power waveform is crushed in the phase direction. As a result, a stable optical output cannot be obtained. More specifically, when the bias current I.sub.B is less than the threshold-value current I.sub.th (I.sub.B&lt;I.sub.th), the optical power develops an oscillation delay time t.sub.d. This is represented generally by the following equation: EQU t.sub.d =.tau.s.multidot.log[I.sub.P /(I.sub.P +I.sub.B -I.sub.th)](1)
where .tau.s represents carrier life and I.sub.P denotes the drive current.
When the adjustment of bias current or compensation for temperature is unsuccessful, the relation I.sub.th &gt;I.sub.B is established, the oscillation delay time t.sub.d in the optical power occurs, as shown in FIG. 24, and the optical power waveform is crushed in the phase direction. As a result, a stable optical output can no longer be obtained.
Further, in the conventional semiconductor laser drive, adjustments are required at three locations, namely adjustments of bias current, drive current and reference voltage. Moreover, it is required that adjustments take into consideration both variance in elements and the temperature characteristics. Such adjustments are very difficult to carry out.
Furthermore, a constant-current circuit shown in FIG. 25 performs control of drive current, control of bias current and control of temperature characteristic according to the prior art. The circuit shown in FIG. 25 includes a FET Q, a diode D for temperature compensation and a variable resistor V.sub.R. In accordance with this constant-current circuit, the variable resistor VR is adjusted to adjust the drain current I.sub.D (drive current I.sub.P or bias current I.sub.B), and a fluctuation in optical power that accompanies a change in temperature is compensated for by the diode, as mentioned above. With this constant-current circuit, however, the gate-source voltage V.sub.gs of the FET varies owing to a fluctuation in the power supply and, hence, the drain current I.sub.D (drive current I.sub.P or bias current I.sub.B) changes. FIG. 26 is a diagram illustrating the manner in which the drain current I.sub.D changes with respect to a fluctuation (0-.+-.10%) in power-supply voltage V.sub.SS. This shows a case in which drain current is 20 mA, 10 mA and 5 mA at a power-supply voltage fluctuation of zero. Thus, when drain current, i.e., drive current I.sub.P, varies owing to a fluctuation in power-supply voltage, a problem which arises is a change in the optical output power. Further, if the bias current I.sub.B varies, a problem encountered is that the optical power waveform changes.